Speed controller for model train

The Type ZN419CE servo control
IC from Ferranti is ideal for use in
model railway direct-current
engines. The internal circuit diagram
of this device is shown in here. The IC varies the pulse
width of input pulses in direct proportion
to the displacement of a
potentiometer. It operates with
positive-going input pulses which
can be coupled directly or via a
capacitor to input pin 14. The
advantage of a.c. coupling is that
should a fault occur which causes
the input signal to become a continuous
level, the servo will remain
in its last quiescent position,
whereas with direct coupling the
servo output arm will rotate continuously.

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